1. Field of the Invention
The present invention relates to a thin-film transistor construction and to a method of fabricating it.
2. Description of the Related Art
Thin-film transistors (TFTs) fabricated on a glass substrate or on an insulating surface have been known. TFTs of this type have been developed especially for use in active matrix liquid crystal displays.
In an active matrix liquid crystal display, millions of pixel electrodes are arranged in rows and columns, and TFTs are connected with these pixel electrodes. Electric charges going into and out of the pixel electrodes are controlled by their respective TFTs.
Manufacture of this type of active matrix liquid crystal display needs a technique for fabricating tens of thousands of TFTs on a glass substrate or quartz substrate that is at least several centimeters square.
With today's technique, it is impossible to produce a single-crystal silicon thin film on a glass or quartz substrate that is at least several centimeters square. Accordingly, generally manufactured silicon films are typified by films of amorphous silicon, polycrystalline silicon, and crystallite silicon.
Where an amorphous silicon film is used, the P-channel type cannot be made practical. Also, high-speed operation cannot be accomplished. Therefore, it is impossible to produce from TFTs using an amorphous silicon film a peripheral driver circuit that is required to operate at or above several megahertz.
On the other hand, where a crystalline silicon film typified by polycrystalline and crystallite silicon films is employed, the P-channel TFT can be put into practical use. Consequently, CMOS circuits can be built. In addition, high-speed operations at or above several megahertz are possible. Utilizing these features, a peripheral driver circuit can be integrated with an active matrix circuit on the same substrate.
Yet, TFTs using a crystalline silicon film suffer from reliability problem and characteristic variations. These give rise to a deterioration of the quality of the displayed image.
These reliability and characteristic variation problems are caused by unstable factors contained in the processing step for creating contact holes, as well as unstable factors contained in the state of the crystalline silicon film forming the active layer.
It is generally known that a silicon oxide film is used as an interlayer dielectric film in TFTs. However, the silicon oxide film poses problems as described below.
The silicon oxide film shows a low etch rate during a dry etching process. In order to obtain a practical etch rate, it is necessary to increase the self-bias voltage to about 600 V. This often results in electrostatic discharge damage due to a voltage induced across multilayer metallization when conductive interconnects are formed.
Furthermore, since the etching process is carried out, using an increased self-bias voltage, the etching process tends to be unstable. Hence, it is difficult to secure a sufficient process margin.
For example, it is difficult to taper the end portions of contact holes by devising the etching conditions.
Generally, where the active layer of TFTs is formed, using a crystalline silicon film, it is necessary to terminate the active layer with hydrogen. That is, the dangling bonds of silicon within the crystalline silicon film are neutralized with hydrogen, thus stabilizing the electrical properties.
It is necessary to form an interlayer dielectric film after the formation of the active layer, irrespective of the type of TFTs.
Where a silicon oxide film is used as the interlayer dielectric film, there arises the problem that hydrogen contained in the active layer is easily freed, because there exists only a weak barrier to hydrogen within the silicon oxide film. This immensely contributes to instability of the TFT characteristics.
Where a silicon oxide film is used as the interlayer dielectric film, it is difficult to detect the endpoint of the etching where the etching is a dry etching process. Generally, quartz jigs are used with a holder or stage that holds a substrate.
In this case, during the dry etching process, silicon oxide constituents are released into the etching ambient from the quartz jigs. This makes it difficult to detect the endpoint of the etching of the silicon oxide film.
In particular, the detection of the silicon oxide component within the ambient renders it difficult to detect the endpoint of the etching of the silicon oxide film clearly.
This means that the number of unstable factors in the manufacturing process increases.